What is SOH used for?

SOH (ASCII 1) marks the beginning of a message header in communication protocols. It separates routing/control information from the actual message content. Think of it as the start of the 'envelope' before the 'letter' inside.

Where does SOH appear in real protocols?

It's used in protocols like BISYNC and some industrial/serial standards to indicate where the header section begins. In HL7 healthcare messaging, SOH-like markers delineate message segments. Some serial device protocols still frame messages with SOH.

Is SOH related to HTTP headers?

Conceptually yes — both separate metadata from content. But HTTP uses text-based boundaries (the blank line \r\n\r\n between headers and body) instead of a control byte. SOH was the binary-protocol approach to the same structural problem.

What generates SOH from a keyboard?

Ctrl+A sends SOH (ASCII 1). In modern terminals, Ctrl+A is intercepted for 'move to start of line' (readline/emacs) or as the tmux/screen prefix key, so the raw SOH byte almost never reaches programs.

Start Of Heading

SOH (Start of Heading) is ASCII code point 1 (0x01), Unicode U+0001. It is a C0 control character that preceded a message header on teleprinter links — everything between SOH and the following STX (0x02) was routing or addressing metadata, not payload. In IBM BSC, the SOH-to-STX region carried station addresses and sequence numbers that the receiver processed before the actual data block began. Ctrl+A emits 0x01, which tmux and GNU Screen repurposed as their default command prefix. UTF-8 encodes it as the single byte 0x01; Unicode classifies it as Cc (Control) with the alias START OF HEADING.

Control Character

Technical Details

Code Example

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unsigned char frame[] = {0x01,'A','1',0x02,'O','K'};
// SOH marks header start; STX marks payload start
int is_soh = (frame[0] == 0x01);

Frequently Asked Questions

SOH (ASCII 1) marks the beginning of a message header in communication protocols. It separates routing/control information from the actual message content. Think of it as the start of the 'envelope' before the 'letter' inside.
It's used in protocols like BISYNC and some industrial/serial standards to indicate where the header section begins. In HL7 healthcare messaging, SOH-like markers delineate message segments. Some serial device protocols still frame messages with SOH.
Conceptually yes — both separate metadata from content. But HTTP uses text-based boundaries (the blank line \r\n\r\n between headers and body) instead of a control byte. SOH was the binary-protocol approach to the same structural problem.
Ctrl+A sends SOH (ASCII 1). In modern terminals, Ctrl+A is intercepted for 'move to start of line' (readline/emacs) or as the tmux/screen prefix key, so the raw SOH byte almost never reaches programs.

Start Of Heading

Control Character